Field of the Invention
The present invention relates to a technique for correcting unevenness of exposure caused by external light variation (generally referred to as “flicker”) that occurs when an image capturing apparatus performs shooting.
Description of the Related Art
Accompanying an increase in the sensitivity of digital cameras in recent years, high-speed shutters have been released also in environments using artificial light sources in which flicker occurs. When a high-speed shutter is released in an environment using a flicker light source, variation occurs in the exposure of each frame during continuous shooting due to variation in the external light. Also, if the brightness of the light source varies within one frame during this time, unevenness of exposure will occur in upper and lower portions of the image. Regarding this kind of problem, for example, a method has been proposed in which the gain of the image is raised or lowered after shooting so as to adjust the exposure.
Japanese Patent Laid-Open No. 2008-236808 discloses a shooting system in which exposure is started by resetting all of the pixels of an image sensor at the time of still image shooting, and exposure is stopped by closing a lens shutter. Also, unevenness of exposure is prevented by controlling a program line diagram in advance such that shooting is performed at an exposure time at which unevenness of exposure caused by flicker is not likely to occur.
Japanese Patent Laid-Open No. 2007-053742 discloses a shooting apparatus that shoots a still image using a combination of a focal-plane mechanical second curtain shutter that suspends exposure and an electronic first curtain shutter that travels to start exposure before the mechanical second curtain shutter suspends exposure. Also, a mechanism is described which corrects unevenness of exposure that appears in an image due to this kind of combination of an electronic shutter and a mechanical shutter.
However, if the exposure is adjusted by changing the gain of the image after shooting, blown-out highlights will appear in bright portions, and the S/N ratio will deteriorate in dark portions. Also, if control is performed such that unevenness of exposure caused by flicker does not occur in the program line diagram as disclosed in Japanese Patent Laid-Open No. 2008-236808, it will no longer be possible for a user to perform shooting using an intended shutter speed and diaphragm value as if shooting using a single-lens reflex camera. Also, with the shooting system disclosed in Patent Document 1, in which exposure is started by resetting all of the pixels in an image sensor once during still image shooting and exposure is suspended by closing the lens shutter, there is no mention of unevenness that appears in the top and bottom portions of an image captured using a focal plane shutter.
In Japanese Patent Laid-Open No. 2007-053742, regarding the unevenness of exposure that appears due to the combination of the electronic first curtain shutter and the mechanical second curtain shutter, the travel curve of the electronic first curtain shutter is corrected using parameters determined as shooting conditions, such as the exit pupil distance (PO value) of the lens. For this reason, the unevenness of exposure caused by flicker cannot be corrected.